The invention relates to a method for operating a dual-fuel internal combustion engine having a gas mixer, an intake path and an engine having a number of cylinders and having an injection system, wherein the dual-fuel internal combustion engine is operated, in a first operating state, in diesel operation with diesel and, in a second operating state, in gas operation with gas as fuel. The invention further relates to a control system for the dual-fuel internal combustion engine as and a dual-fuel internal combustion engine. The dual-fuel internal combustion engine further has, preferably but not necessarily, in the intake path, a forced-induction unit and a bypass path for bypassing the forced-induction unit.
This type of dual-fuel internal combustion engine is also termed a multi-fuel internal combustion engine and can also be operated with, in addition to the preferred fuel choice of diesel and gas, a great many other fuels. Usually, a dual-fuel internal combustion engine is operated with one or the other fuel depending on fuel availability.
In particular in the case of gas operation (in certain cases also additionally or alternatively in diesel operation) a dual-fuel internal combustion engine can be operated in ignition jet operation in accordance with the diesel process with external mixing of a gas-air mixture and preferably a diesel ignition jet. Thus, engines of the dual-fuel internal combustion engine are generally constructed on the basis of a diesel engine construction and are amongst the newest technologies, in particular in the field of environmentally friendly application possibilities for large engines. The type of internal combustion engine mentioned in the introduction can in particular also comprise what is termed a high-pressure gas engine with internal mixing, which can deliver, by means of gas injection in ranges of greater than 200 bar in conjunction with a diesel ignition jet, a relatively high specific cylinder power. The ignition jet engine can also be operated with liquid fuel such as diesel or another liquefied fuel such as liquefied natural gas (LNG) or also liquefied petroleum gas (LPG). A dual-fuel internal combustion engine can thus preferably have a gas-diesel engine to form a gas-diesel internal combustion engine.
It is however also possible in principle to provide central mixing for gas operation, for example by means of a gas mixer. Within the context of the present application, reference is made primarily to gas operation in which mixing takes place individually cylinder by cylinder, preferably immediately upstream of the cylinder. Each cylinder may be assigned a separate gas injection valve which is actuated individually cylinder by cylinder. In particular, actuation can take place depending on the operating cycle of a bank of cylinders. Preferably, an ignition jet of liquid fuel can serve for igniting the combustion gas mixture in the cylinder; it is also possible in principle for spark-ignited gas operation to be provided.
Thus, a dual-fuel internal combustion engine can preferably have an injection system which can preferably be controlled electronically and is suitable for various grades of gas, such as biogas or petroleum gas, in liquid form or also for the use of oils such as vegetable oils or the like as liquid fuel. Common rail injection systems but in certain cases also pump-nozzle injection systems with electronic control have proven to be of great use in this context. As mentioned, in gas operation the ignition medium can be added at high pressure to the gaseous fuel proper of the charge mixture or can also be added to the intake duct. Dual-fuel engines operating in gas operation, in particular ignition jet operation, in accordance with the diesel process with external mixing (i.e. mixing outside the cylinder) are overall more flexible in the use of fuel and produce lower emissions. Fields of application are mobile applications, such as marine engines or utility vehicles and heavy goods vehicles, and also static applications such as local power plants which are in particular advantageously to be configured for a variable gas supply. There arises the preferred possibility, not least because of comparatively constant fuel quality, to operate the dual-fuel internal combustion engine either with gas or with liquid fuel such as diesel or liquefied gas.
For example, EP 2 069 627 B1 discloses a dual-fuel internal combustion engine of the general type with a proportioning system for matching a flow of liquefied gas and diesel, wherein excess fuel mixtures are collected and are returned to a mixing chamber for subsequent combustion in a combustion chamber of the engine.
U.S. Pat. No. 6,131,552 discloses, overall, a fuel control system which can regulate the supply of gas to a mixing chamber as a function of a measured operating state of the engine.
Customarily, air consumption is a measure of the gaseous fresh charge of charge air supplied to a combustion engine in a charge mixture, wherein the air consumption also permits monitoring of the quality of the intake system and intake process. The actual air consumption generally represents the ratio of the mass of fresh air in a charge mixture actually supplied to the engine or to a cylinder thereof, during an operating cycle. This real mixture mass is compared to the theoretical fresh charge mass, determined from the geometric displacement volume and the theoretical charge density under atmospheric conditions (in the case of naturally aspirated engines) or, in the case of forced-induction engines, in this case the state of the fresh charge downstream of the compressor or downstream of the charge air cooler is taken into account.
Fresh charge supplied to a cylinder is affected by a number of factors such as the valve control times or the opening cross section of the valves. This can in principle be determined by a module for determining the engine forced induction, which is supported by an intake path model. In actual fact, however, only in exceptional cases does the fresh charge supplied to the engine in a charge mixture correspond to the theoretical. Air consumption is not a constant value for an engine, but is greatly dependent on the engine speed and the actual geometric ratios of the intake tract and the combustion space; in order to overcome this dependency it is possible to refer for example to a suitable characteristic map.
However, intake path models are in principle known in engine controllers only in the context of general internal combustion engines, such as in EP 1 398 490 A2. Common to these is the widely adopted fundamental concept for modeling the intake path—in the simplest case as a homogeneous pressurized container and in order to detect the dynamic processes in the air path—by modeling the storage behavior of the intake path, also termed suction tube, by means of the filling and emptying methods. In that context, the suction tube is treated as a pressurized container which is continuously filled with air via a throttle flap and out of which the engine sucks air via the inlet valve by means of its suction behavior corresponding to the working rhythm.
In the operation of gas engines, the mixture can as mentioned be formed upstream of the compressor of the exhaust-gas turbocharging and/or also individually cylinder by cylinder upstream of a cylinder. At the same time, the intake path between the compressor outlet and the combustion space inlet consists of partially large volumes which can thus store or discharge significant masses of charge fluid, i.e. in particular masses of mixture or also only masses of charge air. This is in particular the case when, in the event of changes in load on the engine and/or in engine speed, there arise in the individual partial volumes changes in pressure and/or in temperature.
Independently of the manner in which the mixture is formed, it is clear that the fuel supply of a dual-fuel internal combustion engine, in particular in the transient operating range of the internal combustion engine and in the case of variable fuel qualities, and/or a reliable indication of state parameters of a charge fluid in the intake path, is extremely complex. In particular, it is found that operation in the low-load range can be problematic in the case of dual-fuel internal combustion engines, in particular for designing an ignition jet engine.
The above-mentioned control method of U.S. Pat. No. 6,131,552 A proves insufficient for solving the problem, even in the case of complex control systems. This is also the case for other load-dependent gas metering systems or other fuel metering systems in complex control systems; in particular if in addition, especially in the low-load range, hydrocarbon emissions (HC emissions) should be kept as low as possible. It is desirable to provide a more advantageous dual-fuel operation of an internal combustion engine in particular in the transient, preferably low-load range, in accordance with the load requirements and also the emissions requirements.